- Email: [email protected]
A high-voltage test for the ATLAS RPC qualification
ARTICLE IN PRESS
Nuclear Instruments and Methods in Physics Research A 533 (2004) 199–202 www.elsevier.com/locate/nima
A high-voltage test for the ATLAS RPC qualification G. Aielli, P. Camarri, R. Cardarelli, A. Di Ciaccio, A. Di Simone, B. Liberti, R. Santonico INFN Roma 2 and University of Roma ‘‘Tor Vergata’’, Italy Available online 25 July 2004
Abstract The RPC production sequence for the ATLAS experiment includes a specific test of current absorption at the operating point, which concerns the RPC ‘‘gas volumes’’, namely the bare detectors not yet assembled with the read-out panels and the mechanical support structures. The test, which is carried out at the production site, consists of two phases. The gas volumes are initially conditioned with pure argon, keeping the voltage constant just above the breakdown value of about 2 kV. The final test, performed after the volumes have undergone inner surface varnishing with linseed oil, is based on the measurement of the current–voltage characteristics with the binary operating gas, C2 H2 F4 =i-C4 H10 ¼ 95=5. The results presented here concern 45% of the total foreseen production. r 2004 Elsevier B.V. All rights reserved. PACS: 29.40.Cs; 68.47.Mn; 72.80.Tm Keywords: Gaseous detectors; RPC; Plastic laminates
1. Introduction The assembly of the Resistive Plate Chambers for the ATLAS Muon Trigger detector [1] requires the manufacturing of about 3500 gas volumes. For a long-term reliable operation, a certification of each gas volume is required before assembling the RPC units. The ATLAS Corresponding author.
E-mail address: [email protected] (P. Camarri).
RPCs are made of phenolic–melaminic plastic laminates. Here we describe the test procedure and the results obtained in the initial phase of the production (March 2002–October 2003); six different types of RPC have been tested so far: BOL-B (111 242:5 cm2 ), BOS-B (111 183 cm2 ), BML-D (87 172 cm2 ), BML-A (120 172 cm2 ), BML-E (75 172 cm2 ) and BMS-E (75 148 cm2 ). Here we follow the ATLAS naming convention (B for barrel; O/M for outer/middle; L/S for long/short; the last letter
0168-9002/$ - see front matter r 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.nima.2004.07.027
ARTICLE IN PRESS G. Aielli et al. / Nuclear Instruments and Methods in Physics Research A 533 (2004) 199–202
for different transversal sizes). The basic steps of the test are described below.
2. Test set-up and procedure The certification is performed at the gas-volume manufacturing firm (‘‘General Tecnica’’, Italy). The gas volumes are packed in bunches of 23–24 units soon after they have been assembled. The test is based on the measurement of the total current flowing through each gas volume, given by the voltage drop across a 100 kO resistor on the highvoltage return line. All the measurements are performed using PC-controlled 12-bit PCI ADC boards. The data-acquisition and monitoring graphical interface, as well as the data storage, have been implemented using LabVIEW [2]. The gas volumes are initially conditioned with pure argon for 2–3 days [3], keeping the high voltage at 2.1–2.2 kV. In this step, the high voltage is provided by a Bertan power supply (4 kV, 40 mA). The initial current is set at 80–100 mA=m2 for each gas volume, by setting the resistance value on the HV return lines using variable resistors. A continuous drop of the total current (even by a factor larger than 2 at the end) is observed during the first day of the treatment; after that the current becomes stable, following the environment temperature and pressure changes. This procedure is meant to optimize the inner surfaces of the RPC gap before the oil varnishing. The current–voltage (I–V) characteristics for a set
1000
Current (µA)
100 10 1 1500 1600 1700 1800 1900 2000 2100 2200 2300 0.1 0.01
(a)
HV (Volts)
Serie1 Serie2 Serie3 Serie4 Serie5 Serie6 Serie7 Serie8 Serie9 Serie10 Serie11 Serie12 Serie13 Serie14 Serie15 Serie16 Serie17 Serie18 Serie19 Serie20 Serie21 Serie22 Serie23
of 23 gas volumes before and after the conditioning are shown in Fig. 1(a) and (b) respectively. The second step is the gas-volume certification: it is based on the measurement of the I–V characteristics with a binary gas mixture (C2 H2 F4 =i-C4 H10 ¼ 95=5), similar to the one used with the ATLAS RPCs, but without SF6 . Indeed, an amount of 0.3% SF6 is used in the ATLAS gas mixture as a streamer suppressor [4]. On the contrary, in this test the presence of streamers helps enhancing the currents and identifying highcurrent gas volumes. An acceptable gas volume is required to have a measured current lower than 2 mA=m2 in standard operating conditions (applied voltage: 9 kV; temperature: 20 C; pressure: 1000 mbar). The global I–V behavior is also important for the certification, since it may provide evidence for possible gas-volume defects: for instance, a gas volume with comparatively high current at low voltage (lower than 6 kV) is considered defective even though its current at 9 kV is within the limit mentioned above. The I–V characteristics for the same set of gas volumes already described in Fig. 1 are shown in Fig. 2; three gas volumes did not fulfil the acceptance criteria.
3. Recovery of defective gas volumes The gas volumes which fail the qualification test described above undergo a second conditioning cycle. In this case the currents are kept below
1000
100
Current (µA)
200
10 1 1500 1600 1700 1800 1900 2000 2100 2200 2300 0.1 0.01
(b)
HV (Volts)
Serie1 Serie2 Serie3 Serie4 Serie5 Serie6 Serie7 Serie8 Serie9 Serie10 Serie11 Serie12 Serie13 Serie14 Serie15 Serie16 Serie17 Serie18 Serie19 Serie20 Serie21 Serie22 Serie23
Fig. 1. (a) Pure argon current–voltage characteristics for a bunch of BML-A gas volumes before the conditioning. (b) Argon ðI; V Þ characteristics for the same gas-bunch after the conditioning.
ARTICLE IN PRESS G. Aielli et al. / Nuclear Instruments and Methods in Physics Research A 533 (2004) 199–202
201
6 Serie1
rejected gas volumes
5
Serie2 Serie3 Serie4 Serie5 Serie6
4
Current (µA)
Serie7 Serie8
Requirement: I9 < 4 µA
3
Serie9 Serie10 Serie11 Serie12
2
Serie13 Serie14 Serie15 Serie16
1
Serie17 Serie18 Serie21
0
Serie22
0
1000
2000
3000
4000
5000
6000
7000
8000
9000
10000
Serie23
HV (Volts) Fig. 2. Binary mixture current–voltage characteristics for the same bunch of BML-A gas volumes referred to in Fig. 1. Three gas volumes not satisfying the acceptance criteria are evidenced.
35
90 80
30
70 25
60
20
50
15
40 30
10
20 5
10
0
0 0.4 1.2
(a)
2
2.8 3.6 4.4 5.2 6 ρ20(1010 Ω
6.8 7.6 8.4 9.2
0.4 1.2
10
cm)
(b)
2
2.8 3.6 4.4 5.2 6 ρ20(1010 Ω
6.8 7.6 8.4 9.2
10
cm)
Fig. 3. Average r20 distributions for the tested bunches of BML-A (a) and BML-E (b) gas volumes.
30 mA=m2 not to damage the inner surfaces that are already varnished. Then the acceptance criteria are checked again. The gas volumes failing also the second acceptance test are definitively rejected. The average fraction of gas volumes recovered after the second conditioning is about 52%. However, this value is not uniformly distributed among the various gas-volume bunches. The recovery step for the gas volumes tested so far has been extremely effective for the BML-D, BML-A and BMS-E, but ineffective for the
BML-E bunches. This may be related to specific problems shown by the BML-E bunches, such as bad spacer gluing and average plate resistivity out of the specifications for many gas volumes, as discussed in the following section.
4. Average plate resistivity The ðI; V Þ characteristics measured in the conditioning procedure described above also
ARTICLE IN PRESS 202
G. Aielli et al. / Nuclear Instruments and Methods in Physics Research A 533 (2004) 199–202
provide a way of measuring the plate resistivity of an assembled gas volume. Indeed, the voltage– current characteristics above 2.2 kV show a linear behavior, which means that the voltage drop DV across the gas gap is fixed in this condition, and the current increase DI is only limited by the total plate resistance, R ¼ DV =DI. The average plate resistivity is given by r ¼ ðS=2dÞR, where S is the area of the gas gap, and d is the plate thickness. In order to fix common reference conditions for all the measurements, the resistivity at 20 C is evaluated using an empirical scaling law: r20 ¼ rðTÞ eðT20Þ=8:1 , where T is expressed in C. Fig. 3 shows the average r20 distributions for two bunches of tested gas volumes. The distribution in Fig. 3(a) essentially fits the ATLAS specifications, while the one in Fig. 3(b) shows a low-resistivity peak which is out of the ATLAS accepted range.
5. Conclusions A gas-volume quality control procedure has been planned to certify the RPCs for the ATLAS Muon Trigger Detector. The present acceptance for the ATLAS RPC gas volumes, based on a statistics of about 1500 tests, is 86%. A lower acceptance ratio observed for a gas-volume bunch (BML-E type) was shown to be correlated to specific problems with the plate surface.
References [1] ATLAS Muon Collaboration, ATLAS Muon Spectrometer—Technical Design Report, CERN/LHCC 97–22, 5 June 1997. [2] National Instruments, LabVIEW User Manual, November 2001 ed. [3] G. Aielli, et al., Nucl. Instr. and Meth. A 515 (2003) 335. [4] P. Camarri, et al., Nucl. Instr. and Meth. A 414 (1998) 317.
Nuclear Instruments and Methods in Physics Research A 533 (2004) 199–202 www.elsevier.com/locate/nima
A high-voltage test for the ATLAS RPC qualification G. Aielli, P. Camarri, R. Cardarelli, A. Di Ciaccio, A. Di Simone, B. Liberti, R. Santonico INFN Roma 2 and University of Roma ‘‘Tor Vergata’’, Italy Available online 25 July 2004
Abstract The RPC production sequence for the ATLAS experiment includes a specific test of current absorption at the operating point, which concerns the RPC ‘‘gas volumes’’, namely the bare detectors not yet assembled with the read-out panels and the mechanical support structures. The test, which is carried out at the production site, consists of two phases. The gas volumes are initially conditioned with pure argon, keeping the voltage constant just above the breakdown value of about 2 kV. The final test, performed after the volumes have undergone inner surface varnishing with linseed oil, is based on the measurement of the current–voltage characteristics with the binary operating gas, C2 H2 F4 =i-C4 H10 ¼ 95=5. The results presented here concern 45% of the total foreseen production. r 2004 Elsevier B.V. All rights reserved. PACS: 29.40.Cs; 68.47.Mn; 72.80.Tm Keywords: Gaseous detectors; RPC; Plastic laminates
1. Introduction The assembly of the Resistive Plate Chambers for the ATLAS Muon Trigger detector [1] requires the manufacturing of about 3500 gas volumes. For a long-term reliable operation, a certification of each gas volume is required before assembling the RPC units. The ATLAS Corresponding author.
E-mail address: [email protected] (P. Camarri).
RPCs are made of phenolic–melaminic plastic laminates. Here we describe the test procedure and the results obtained in the initial phase of the production (March 2002–October 2003); six different types of RPC have been tested so far: BOL-B (111 242:5 cm2 ), BOS-B (111 183 cm2 ), BML-D (87 172 cm2 ), BML-A (120 172 cm2 ), BML-E (75 172 cm2 ) and BMS-E (75 148 cm2 ). Here we follow the ATLAS naming convention (B for barrel; O/M for outer/middle; L/S for long/short; the last letter
0168-9002/$ - see front matter r 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.nima.2004.07.027
ARTICLE IN PRESS G. Aielli et al. / Nuclear Instruments and Methods in Physics Research A 533 (2004) 199–202
for different transversal sizes). The basic steps of the test are described below.
2. Test set-up and procedure The certification is performed at the gas-volume manufacturing firm (‘‘General Tecnica’’, Italy). The gas volumes are packed in bunches of 23–24 units soon after they have been assembled. The test is based on the measurement of the total current flowing through each gas volume, given by the voltage drop across a 100 kO resistor on the highvoltage return line. All the measurements are performed using PC-controlled 12-bit PCI ADC boards. The data-acquisition and monitoring graphical interface, as well as the data storage, have been implemented using LabVIEW [2]. The gas volumes are initially conditioned with pure argon for 2–3 days [3], keeping the high voltage at 2.1–2.2 kV. In this step, the high voltage is provided by a Bertan power supply (4 kV, 40 mA). The initial current is set at 80–100 mA=m2 for each gas volume, by setting the resistance value on the HV return lines using variable resistors. A continuous drop of the total current (even by a factor larger than 2 at the end) is observed during the first day of the treatment; after that the current becomes stable, following the environment temperature and pressure changes. This procedure is meant to optimize the inner surfaces of the RPC gap before the oil varnishing. The current–voltage (I–V) characteristics for a set
1000
Current (µA)
100 10 1 1500 1600 1700 1800 1900 2000 2100 2200 2300 0.1 0.01
(a)
HV (Volts)
Serie1 Serie2 Serie3 Serie4 Serie5 Serie6 Serie7 Serie8 Serie9 Serie10 Serie11 Serie12 Serie13 Serie14 Serie15 Serie16 Serie17 Serie18 Serie19 Serie20 Serie21 Serie22 Serie23
of 23 gas volumes before and after the conditioning are shown in Fig. 1(a) and (b) respectively. The second step is the gas-volume certification: it is based on the measurement of the I–V characteristics with a binary gas mixture (C2 H2 F4 =i-C4 H10 ¼ 95=5), similar to the one used with the ATLAS RPCs, but without SF6 . Indeed, an amount of 0.3% SF6 is used in the ATLAS gas mixture as a streamer suppressor [4]. On the contrary, in this test the presence of streamers helps enhancing the currents and identifying highcurrent gas volumes. An acceptable gas volume is required to have a measured current lower than 2 mA=m2 in standard operating conditions (applied voltage: 9 kV; temperature: 20 C; pressure: 1000 mbar). The global I–V behavior is also important for the certification, since it may provide evidence for possible gas-volume defects: for instance, a gas volume with comparatively high current at low voltage (lower than 6 kV) is considered defective even though its current at 9 kV is within the limit mentioned above. The I–V characteristics for the same set of gas volumes already described in Fig. 1 are shown in Fig. 2; three gas volumes did not fulfil the acceptance criteria.
3. Recovery of defective gas volumes The gas volumes which fail the qualification test described above undergo a second conditioning cycle. In this case the currents are kept below
1000
100
Current (µA)
200
10 1 1500 1600 1700 1800 1900 2000 2100 2200 2300 0.1 0.01
(b)
HV (Volts)
Serie1 Serie2 Serie3 Serie4 Serie5 Serie6 Serie7 Serie8 Serie9 Serie10 Serie11 Serie12 Serie13 Serie14 Serie15 Serie16 Serie17 Serie18 Serie19 Serie20 Serie21 Serie22 Serie23
Fig. 1. (a) Pure argon current–voltage characteristics for a bunch of BML-A gas volumes before the conditioning. (b) Argon ðI; V Þ characteristics for the same gas-bunch after the conditioning.
ARTICLE IN PRESS G. Aielli et al. / Nuclear Instruments and Methods in Physics Research A 533 (2004) 199–202
201
6 Serie1
rejected gas volumes
5
Serie2 Serie3 Serie4 Serie5 Serie6
4
Current (µA)
Serie7 Serie8
Requirement: I9 < 4 µA
3
Serie9 Serie10 Serie11 Serie12
2
Serie13 Serie14 Serie15 Serie16
1
Serie17 Serie18 Serie21
0
Serie22
0
1000
2000
3000
4000
5000
6000
7000
8000
9000
10000
Serie23
HV (Volts) Fig. 2. Binary mixture current–voltage characteristics for the same bunch of BML-A gas volumes referred to in Fig. 1. Three gas volumes not satisfying the acceptance criteria are evidenced.
35
90 80
30
70 25
60
20
50
15
40 30
10
20 5
10
0
0 0.4 1.2
(a)
2
2.8 3.6 4.4 5.2 6 ρ20(1010 Ω
6.8 7.6 8.4 9.2
0.4 1.2
10
cm)
(b)
2
2.8 3.6 4.4 5.2 6 ρ20(1010 Ω
6.8 7.6 8.4 9.2
10
cm)
Fig. 3. Average r20 distributions for the tested bunches of BML-A (a) and BML-E (b) gas volumes.
30 mA=m2 not to damage the inner surfaces that are already varnished. Then the acceptance criteria are checked again. The gas volumes failing also the second acceptance test are definitively rejected. The average fraction of gas volumes recovered after the second conditioning is about 52%. However, this value is not uniformly distributed among the various gas-volume bunches. The recovery step for the gas volumes tested so far has been extremely effective for the BML-D, BML-A and BMS-E, but ineffective for the
BML-E bunches. This may be related to specific problems shown by the BML-E bunches, such as bad spacer gluing and average plate resistivity out of the specifications for many gas volumes, as discussed in the following section.
4. Average plate resistivity The ðI; V Þ characteristics measured in the conditioning procedure described above also
ARTICLE IN PRESS 202
G. Aielli et al. / Nuclear Instruments and Methods in Physics Research A 533 (2004) 199–202
provide a way of measuring the plate resistivity of an assembled gas volume. Indeed, the voltage– current characteristics above 2.2 kV show a linear behavior, which means that the voltage drop DV across the gas gap is fixed in this condition, and the current increase DI is only limited by the total plate resistance, R ¼ DV =DI. The average plate resistivity is given by r ¼ ðS=2dÞR, where S is the area of the gas gap, and d is the plate thickness. In order to fix common reference conditions for all the measurements, the resistivity at 20 C is evaluated using an empirical scaling law: r20 ¼ rðTÞ eðT20Þ=8:1 , where T is expressed in C. Fig. 3 shows the average r20 distributions for two bunches of tested gas volumes. The distribution in Fig. 3(a) essentially fits the ATLAS specifications, while the one in Fig. 3(b) shows a low-resistivity peak which is out of the ATLAS accepted range.
5. Conclusions A gas-volume quality control procedure has been planned to certify the RPCs for the ATLAS Muon Trigger Detector. The present acceptance for the ATLAS RPC gas volumes, based on a statistics of about 1500 tests, is 86%. A lower acceptance ratio observed for a gas-volume bunch (BML-E type) was shown to be correlated to specific problems with the plate surface.
References [1] ATLAS Muon Collaboration, ATLAS Muon Spectrometer—Technical Design Report, CERN/LHCC 97–22, 5 June 1997. [2] National Instruments, LabVIEW User Manual, November 2001 ed. [3] G. Aielli, et al., Nucl. Instr. and Meth. A 515 (2003) 335. [4] P. Camarri, et al., Nucl. Instr. and Meth. A 414 (1998) 317.